Japanese Patent Application Laid-open Publication No. 2004-116131 (PTL 1) discloses a movable breakwater including a plurality of sleeved steel pipes inserted vertically penetrating through the concrete foundation provided at the seabed into the ground thereunder and arranged close together with the top faces thereof open to the concrete foundation surface as well, buoyant steel pipes inserted into the sleeved steel pipes in a manner capable of being raised and lowered as well as having the bottom faces thereof open and the top faces thereof closed, and an air charging system for supplying air into each buoyant steel pipe. With this structure, during a calm, the row of buoyant steel pipes are set under the seabed to leave the outside of the harbor and the harbor basin completely open, and during stormy weather, air is supplied into each buoyant steel pipe using an external air charging system such as a compressor and piping from the shore, making the row of buoyant steel pipes emerge out from the sea by buoyancy to prevent waves from intruding into the harbor basin.